network model(osi & tcpip)

8/9/2019 Network Model(OSI & TCPIP)

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Network Models


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2.1 LAYERED TASKS2.1 LAYERED TASKS

WeWe useuse thethe conceptconcept ofoflayerslayers inin ourour dailydaily lifelife.. AsAs anan example,example,

letlet usus considerconsider twotwo friendsfriends whowho communicatecommunicate throughthrough postalpostal

mailmail. . TheThe processprocess ofof sendingsending aa letterletter toto aa friend friend wouldwould bebe

complexcomplex ifif therethere werewere nono servicesservices availableavailable from from thethe postpost

officeoffice..

Sender, Receiver, and Carrier

Hierarchy

Topics discussed in this section:Topics discussed in this section:


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Layered TasksLayered Tasks

Sender, Receiverand Carrier


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Layered TasksLayered Tasks

Hierarchy

HigherLayer

Middle Layer

LowerLayer

Services

The Each layer uses the services of the layer immediately

below it.


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2.2 THE OSI MODEL2.2 THE OSI MODEL

Established

Established inin 1947 1947, , thethe International International StandardsStandards

OrganizationOrganization ((ISOISO) ) isis aa multinationalmultinational bodybody dedicateddedicated toto

worldwideworldwide agreementagreement onon internationalinternational standardsstandards. . An An ISOISO

standardstandard thatthat coverscovers allall aspectsaspects ofof networknetwork communicationscommunications isis

thethe OpenOpen Systems Systems InterconnectionInterconnection ((OSIOSI) ) modelmodel.. ItIt waswas firstfirst

introducedintroduced inin thethe latelate 19701970ss..

Layered Architecture

Peer-to-Peer Processes

Encapsulation


ISO is the organization.

OSI is the model.


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Layered ArchitectureLayered Architecture

The OSI model is composed of seven layers ;

Physical (layer1), Data link (layer2), Network (layer3)

Transport (layer4), Session (layer5), Presentation (layer6)

Application (layer7)

Layer

Designer identified which networking functions had related

uses and collected those functions into discrete groups that

became the layers.

The OSI model allows complete interoperability betweenotherwise incompatible systems.

The Each layer uses the services of the layer immediately

below it.


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Figure 2.2 Seven layers of the OSI model

Layered Architecture (contd)Layered Architecture (contd)


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PeerPeer--tt -- eerPr essespeerPr esses

Layer x on one machine communicates with layer x onanother machine - called Peer-to-Peer Processes.

Interfaces between Layers

Each interface defines what information and services a layer

must provide for the layerabove it.Well defined interfaces and layer functions provide modularityto a network

Organizations of the layers

Network support layers : Layers 1, 2, 3

User support layer : Layer 5, 6, 7

It allows interoperabilityamong unrelated software systems

Transport layer (Layer 4) : links the two subgroups


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Figure 2.3 The interaction between layers in the OSI model

PeerPeer--toto--peer Processes (contd)peer Processes (contd)


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Figure 2.4 An exchange using the OSI model

The data portion ofa packet at level N-1 carries the whole packet

from level N. The concept is called encapsulation.

PeerPeer--toto--peer Processes (contd)peer Processes (contd)


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2.3 LAYERS IN THE OSI MODEL2.3 LAYERS IN THE OSI MODEL

InIn thisthis sectionsection wewe brieflybriefly describedescribe thethe functionsfunctions ofof eacheach layerlayerinin thethe OSIOSI modelmodel..

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Session Layer

Presentation Layer

Application Layer



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Physical LayerPhysical Layer

Physical layer coordinates the functions required totransmit a bit stream overa physical medium.

The physical layer is responsible for movements of

individual bits from one hop (node) to the next.


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Physical LayerPhysical Layer

Physical layer is concerned with the following:

(deal with the mechanical and electrical specification of

the primary connections: cable, connector)

Physical characteristics of interfaces and medium

Representation of bits

Data rate : transmission rate

Synchronization of bits

Line configuration

Physical topology

Transmission mode


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DataLink LayerDataLink Layer

The data link layer is responsible for movingframes from one hop (node) to the next.


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Major duties

Framing

Physical addressing

Flow control

Error control

Access control


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Hop-to-hop (node-to-node) delivery


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Network LayerNetwork Layer

The network layer is responsible for the delivery of

individual packets from the source host to the destination

host.


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Logical addressing

Routing


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Transport LayerTransport Layer

The transport layer is responsible for the delivery

ofa message from one process to another.


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Service port addressing

Segmentation and reassembly

Connection control

Flow control

Error control


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Session LayerSession Layer

The session layer is responsible for dialog control and

synchronization.


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Presentation LayerPresentation Layer

The presentation layer is responsible for translation,compression, and encryption


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Application LayerApplication Layer

The application layer is responsible for providingservices to the user.


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The major duties of the application Network virtual terminal

File transfer, access, and management

Mail services

Directory services


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Figure 2.15 Summary of layers

Summary ofLayersSummary ofLayers


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2.4 TCP/IP PROTOCOL SUITE2.4 TCP/IP PROTOCOL SUITE

TheThe layerslayers inin thethe TCP/IPTCP/IP protocolprotocol suitesuite dodo notnot exactlyexactly matchmatchthosethose inin thethe OSIOSI modelmodel.. TheThe originaloriginal TCP/IPTCP/IP protocolprotocol suitesuite

waswas defineddefined asas havinghaving fourfour layerslayers:: hosthost--toto--network,network, internetinternet,,

transport,transport, andand applicationapplication.. However, However, whenwhen TCP/IPTCP/IP isis

comparedcompared toto OSI,OSI, wewe cancan saysay thatthat thethe TCP/IPTCP/IP protocolprotocol suitesuite isismademade ofof fivefive layerslayers:: physical,physical, datadata link,link, network,network, transporttransport,,

andandapplicationapplication..

Physical and Data Link Layers

Network Layer

Transport Layer

Application Layer



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Figure 2.16 TCP/IP and OSI model

TCP/IP Protocol SuiteTCP/IP Protocol Suite


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Physical and DataLink LayersPhysical and DataLink Layers

At the physical and data link layers, TCP/

IP does not

define any specific protocol.

It supports all the standard and proprietary protocols.

A network in a TCP/IP internetwork can be a local-area

network ora wide-area network.


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TCP/

IP supports the

Internetworking Protocol.

IP uses four supporting protocols : ARP, RARP, ICMP,

and IGMP.

IP (Internetworking Protocol)

ARP (Address Resolution Protocol)

RARP (Reverse Address Resolution Protocol)

ICMP (Internet Control Message Protocol)

IGMP (Internet Group Message Protocol)


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The transport layer was represented in TCP/

IP by two

protocols : TCP and UDP.

IP is a host-to-host protocol

TCP and UDP are transport level protocols

responsible for delivery ofa message from aprocess to another process.

UDP (User Datagram Protocol)

TCP (Transmission Control Protocol)

SCTP (Stream Control Transmission Protocol)


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The application layer in TCP/

IP is equivalent to the

combined session, presentation, and application layers

in the OSI model.

Many protocols are defined at this layer.


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22--5 ADDRESSING5 ADDRESSING

FourFour levelslevels ofof addressesaddresses areare usedused inin anan internetinternet employingemploying thetheTCP/IPTCP/IP protocolsprotocols:: physicalphysical,, logicallogical,, portport,, andandspecificspecific..

Physical Addresses

Logical Addresses

Port AddressesSpecific Addresses

Topics discussed in this sectionTopics discussed in this section::


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Figure 2.17 Addresses in TCP/IP

AddressesAddresses


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Figure 2.18 Relationship of layers and addresses in TCP/IP

Physical AddressesPhysical Addresses


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Physical AddressesPhysical Addresses

The physical address, also known as the link address,is the address ofa node as defined by its LAN or WAN.

It is included in the frame used by the data link layer.

The physical addresses have authority over the network (LAN

or WAN).

The size and format of these addresses vary depending on the

network.


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In Figure 2.19 a node with physical address 10 sends a frame

to a node with physical address 87. The two nodes are

connected by a link (bus topology LAN). As the figure shows,

the computer with physical address 10 is the sender, and the

computer with physical address 87is the receiver.

Example 2.1

Physical Addresses (contd)Physical Addresses (contd)


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Figure 2.19 Physical addresses



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As we will see in Chapter 13, most local-area networks use a

48-bit (6-byte) physical address written as 12 hexadecimal

digits; every byte (2 hexadecimal digits) is separated by acolon, as shown below:

Example 2.2

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.



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Logical AddressesLogical Addresses

Logical addresses are necessary for universalcommunications that are independent of underlying

physical networks.

Physical addresses are not adequate in an internetwork

environment where different networks can have different

address formats.

A universal addressing system is needed in which host can be

identified uniquely, regardless of the underlying physical

network.


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Figure 2.20 shows a part of an internet with two routers

connecting three LANs.E

ach device (computer or router) hasa pair of addresses (logical and physical) for each

connection. In this case, each computer is connected to only

one link and therefore has only one pair of addresses. Each

router, however, is connected to three networks (only two are

shown in the figure). So each router has three pairs of

addresses, one for each connection.

Example 2.3

Logical Addresses (contd)Logical Addresses (contd)


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Figure 2.20 IP addresses

The physical addresses will change from hop to hop,

but the logical addresses usually remain the same.

Logical Addresses (contd)Logical Addresses (contd)


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Port AddressesPort Addresses

The

IP and the physical address are necessary fora

quantity of data to travel from a source to the

destination host.

The end object ofInternet communication is a process

communicating with another process.

For these processes to receive data simultaneously, we

need a method to label assigned to a process is called

a port address.

A port address in TCP/IP is 16 bits in length.


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Figure 2.21 shows two computers communicating via the

Internet. The sending computer is running three processes at

this time with port addresses a, b, and c. The receivingcomputer is running two processes at this time with port

addresses j and k. Process a in the sending computer needs to

communicate with process j in the receiving computer. Note

that although physical addresses change from hop to hop,

logical and port addresses remain the same from the source

to destination.

Example 2.4

Port Addresses (contd)Port Addresses (contd)


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Figure 2.21 Port addresses

The physical addresses will change from hop to hop,

but the logical and port addresses usually remain the same.



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Example 2.5

As we will see in Chapter 23, a port address is a 16-bitaddress represented by one decimal number as shown.

753

A 16-bit port address representedas one single number.



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Specific AddressesSpecific Addresses

Some applications have user-friendlyaddresses that

are designed for that specific address.

E-mail address

URL (Universal Resource Locator)


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